At the present time, it is usually required to have two people to test the functions of a trailer light configuration, one being required to stand behind the trailer and the other being required to operate the controls within the cabin of a vehicle (for example a truck), including the turning lamps, the tail light and the brake light. Particularly with the brake light, it is not reasonably feasible for a single operator to ascertain whether or not his brake lights are functioning.
However, there is frequently a requirement to test lamps of a number of trailer vehicles which may be in a yard, and otherwise ready to depart. In such cases, the time delays associated with such testing are unacceptable. The alternative used has been for a large battery to be wheeled from trailer to trailer, and the function of each trailer lamp circuit to be separately tested. This alternative is still unacceptably slow, and often very inconvenient.
In most countries however it is a legal requirement that all trailer lights should function, and therefore it is necessary for tests to be undertaken, usually before each time a trailer is used.
Problems with circuits of various types have been recognized, and reference may be made to Australian Patent 488353 (Lansing Bagnail) dealing with monitoring current leakage in electric trucks, Australian application 20575/76 (Deere & Company), dealing with determination of continuity in an electric circuit, and Australian Patent 495113 (General Signal Corporation) dealing with ground fault detection. Other less relevant prior art includes Australian Patents 545161 (Lockheed), 501552 (Westinghouse), 497189 (Secheron), Application 87806/91 (British Aerospace) and 585846 (Glover).
None of these however employs the combination of separate testing of the circuits of a trailer by energising those circuits through a lamp or lamps, in addition to testing a tractor plug to ensure existence of available energy for the circuits.